1. Field of the Invention
The present invention relates in general to a drive system of a hybrid vehicle, and more particularly to techniques for improving fuel economy of a hybrid vehicle drive system
2. Discussion of Prior Art
JP-2000-197208A discloses a drive system of a hybrid vehicle, which includes an engine, a first electric motor, a second electric motor, and a planetary gear mechanism having three rotary elements. The planetary gear mechanism has a first rotary element in the form of a carrier connected to the engine, a second rotary element in the form of a sun gear connected to the first electric motor, and a third rotary element in the form of a ring ear connected to the second electric motor and drive wheels of the hybrid vehicle. A major portion of the engine output is mechanically transmitted through the planetary gear mechanism to the drive wheels, while a portion of the engine output is converted into an electric energy by the first electric motor operable as an electric generator. This electric energy is supplied through an inverter to the second electric motor to operate the second electric motor for rotating the drive wheels. The electric energy may be stored in an electric-energy storage device through the inverter, and the electric energy stored in the electric-energy storage device can be supplied to the first electric motor M1 through the inverter, to operate the first electric motor for rotating the drive wheels. The speed ratio of the planetary gear mechanism (ratio of the rotating speed of the first rotary element to the rotating speed of the third rotary element) is continuously variable when the operating state of the first electric motor is controlled.
The hybrid vehicle drive system disclosed in the above-identified publication JP-2000-197208A has a power transmitting path consisting of a mechanical path through which the output of the engine is mechanically transmitted through the planetary gear mechanism to the drive wheels, and an electric path through which the electric energy generated by the first electric motor operated by a portion of the engine output is supplied to the second electric motor or back to the first electric motor, to convert the electric energy into a mechanical energy for rotating the drive wheels. The power transmitting efficiency of the mechanical path does not change considerably even when the speed ratio of the planetary gear mechanism changes. In this drive system, the operating speed of the engine can be controlled owing to the differential function of the planetary gear mechanism, so as to maximize the fuel economy of the engine, without an influence by the running speed of the hybrid vehicle. Accordingly, the fuel economy of the hybrid vehicle can be improved owing to the power transmitting efficiency of the electric path that can be held at a high value in spite of a change of the speed ratio of the planetary gear mechanism. In the hybrid vehicle drive system in question, however, the connections of the engine, first electric motor and drive wheels to the rotary elements of the planetary gear mechanism in the form of the carrier, sun gear and ring gear cannot be changed, so that the relationship between the power transmitting efficiency of the electric path and the change of the speed ratio of the planetary gear mechanism cannot be changed considerably, whereby it is not possible to maintain a high degree of efficiency of power transmission from the engine to the drive wheels, over a wide range of change of the speed ratio of the planetary gear mechanism.